The present invention relates generally to an integrated atmospheric transverse coherence length/laser radiation angle-of-arrival measurement system, to record the quality of the atmospheric channel during testing of free space laser communications systems. The laser radiation angle-of-arrival measurements aid in the optical system alignment of the system as well as serve as a record of atmospheric turbulence and platform induced angle-of-arrival changes of the laser communications signal.
One of the most important items to consider when using free space laser communications in an atmospheric environment is the atmosphere. Whether communication is from air-to-air or air-to-ground, the atmosphere plays a major role in corrupting it. Moisture, aerosols, temperature and pressure changes produce refractive index variations in the air by causing random variations in density. These variations, depicted in FIG. 1, are referred to as eddies and have a lens effect on light passing through them. When a plane wave passes through these eddies, parts of it are refracted randomly causing a distorted wavefront with the combined effects of variation of intensity across the wavefront and warping of the isophase surface. By the time the light reaches its destination, it is no longer spatially or temporally coherent over the entire wavefront, and an optical system sampling a large portion of the wavefront would not be able to focus the light to the diffraction limit of the optics. Instead, the size of the airy disk produced would be a function of the diffraction limited aperture of the atmosphere, or Atmospheric Transverse coherence length, r.sub.o, as it is sometimes called. An optical system which has an aperture equal to or less than the r.sub.o will sample a coherent portion of the wave and produce an image that is based upon the quality of the optical equipment. If the aperture of the optical equipment is larger than the r.sub.o, then the quality of the image will be dependent upon the amount of atmospheric turbulence in the optical path.
An airy disk is defined as the bright spot in the system of diffraction rings formed by an optical system with light from a point's source (as a star).
One of the earliest methods of determining r.sub.o involved photography. Light from a distant star was focused onto a photographic plate and after a suitable exposure time the plate was developed. The diameter of the focused spot was then determined with the aid of a densitometer and a mechanical measuring device. The r.sub.o was then calculated in accordance with diffraction theory and the following equations which relate the telescope aperture, D, the wavelength of light, .lambda., and the focal length of the optical system, f.l., to the size of the focused spot, airy disk diameter: EQU AiryDiskDiameter=2.44(.lambda.* f.l.)/D
Knowing that using an optical system with an aperture greater than the r.sub.o will produce a focused spot size which is dependent upon the diffraction limited aperture of the atmosphere allows us to replace the telescope aperture, D, in the diffraction equation with r.sub.o to arrive at a relationship between the Airy Disk Diameter and r.sub.o : ##EQU1##
Even though one could conceivably use an automatic camera to take a sequence of pictures over a period of time, it would be a rather slow process and in no way could it be considered close to a real-time measurement. As interest in the stochastic nature of the atmosphere grew and technology advanced, other methods of measuring the "seeing condition" of the atmosphere were developed.
A past endeavor of mine involved developing a method of measuring the r.sub.o for use as a tool to help characterize the atmosphere for surveillance applications. I used a 1-meter aperture cassegrain telescope, shown in FIG. 2, to gather incoming light from a point source and focus it onto a spinning reticle wheel which contained a track of apertures which increased geometrically in size (ref. Wilkins, Technical Report RADC-TR-86-192, titled "Measurement of the Atmospheric Phase Coherence Length, r.sub.o " Rome Air Development Center, Air Force Systems Command, Griffis Air Force Base, N.Y. 13441-5700, 1986). Light passing through the apertures of the reticle was collected by a photomultiplier tube and the resulting electrical signal was digitized by an analog-to-digital converter. The digital output contained information about the modulation transfer function of the atmosphere and was reduced by computer to provide the diffraction limited aperture of the atmosphere. The system was large, even without the 1-meter telescope, and the optics were difficult to keep aligned due to atmospheric turbulence induced beam wander. Although the system was good for its time, its size and mechanical parts were impractical for use as a real time atmospheric turbulence monitor for laser communications systems.
Other methods of obtaining the r.sub.o have also been employed. One such method is to measure the refractive index structure parameter, C.sub.n.sup.2, using a stellar scintillometer. This method involves taking several scintillation measurements along the optical path and deriving the C.sub.n.sup.2 information analytically. The r.sub.o can then be obtained by using an equation which relates r.sub.o to C.sub.n.sup.2. Although this method does arrive at the r.sub.o, it was not considered acceptable for our laser communications work because it is time intensive. Ostensibly, atmospheric conditions change due to wind, temperature and pressure changes. Consequently, the atmospheric refractive index structure parameter and thus the diffraction limited aperture of the atmosphere also undergo constant change. Since, using the C.sub.n.sup.2 method requires several scintillation measurements and analytical computations for each r.sub.o measurement, it is not possible to derive the r.sub.o fast enough to make the required communications parameter changes to allow for an optimum communication channel. The following United States patents and SIR, relating to angle-of-arrival sensors, are of interest.
4,880,305--Salt PA1 4,824,245--Gardner et al
H 412--Miller, Jr et al
4,498,768--Holl
The Salt patent describes a detector for use in determining the orientation of a laser beam. The detector comprises a fiber optic bundle with a polished input end disposed at a known orientation and an output end, and a sensor adjacent the output end of the bundle. In operation, the sensor measures the diameter of a light output from the fiber optic bundle at a predetermined plane perpendicular to the longitudinal axis of the fiber optic bundle. The diameter varies as a function of the angle of the incidence of the laser beam on the input end.
The Gardner et al patent describes a response ratioing angle of arrival sensor. The invention includes a sensor assembly having two detectors per plane of measurement, and means for dividing the incident electromagnetic radiation into first and second components. The components have intensities determined by angle of arrival of the incident radiation. The first and second detectors are respectively responsive to the first and second radiation components, and provide outputs which are indicative of the angle of arrival of the incident electromagnetic radiation.
The Miller, Jr et al SIR is directed to a device which when illuminated by optical radiation will determine the direction from which the radiation originated. The device comprises a linear detector array and an opaque mask having a narrow slot. The array is positioned a fixed distance behind the opaque mask, and oriented perpendicular to the slot. Light from the laser source, limited by the mask and slot, falls on only a few adjacent elements of the detector array, depending on the direction of arrival of the light.
The Holl patent describes an apparatus for measuring the horizontal and vertical aspect angles of radiation received from a remote laser transmitter. The apparatus comprises a triangular cube corner reflector with built-in linear arrays, and an electronic processor. The detectors are linear arrays along the three edges of the entrance aperture. In operation, the processor receives signals indicating the number and location of the detectors which have been illuminated by the incoming radiation, and determines the horizontal and vertical aspect angles of the radiation.